1. Field of the Invention
The present invention relates to an oxide semiconductor film, a method for forming the oxide semiconductor film, a semiconductor device, and a method for manufacturing the semiconductor device.
Note that in this specification, a semiconductor device refers to any device that can function by utilizing semiconductor characteristics, and an electro-optical device, a semiconductor circuit, an electronic device, and the like are all semiconductor devices.
2. Description of the Related Art
A technique for forming a transistor by using a semiconductor film formed over a substrate having an insulating surface has attracted attention. The transistor is applied to a wide range of semiconductor devices such as an integrated circuit and a display device. A silicon film is known as a semiconductor film applicable to a transistor.
Whether an amorphous silicon film or a polycrystalline silicon film is used as a silicon film in a transistor depends on the purpose. For example, in the case of a transistor included in a large-sized display device, it is preferred to use an amorphous silicon film, which can be formed using the established technique for forming a film on a large-sized substrate. On the other hand, in the case of a transistor included in a high-performance display device where driver circuits are formed over the same substrate, it is preferred to use a polycrystalline silicon film, which can form a transistor having a high field-effect mobility. As a method for forming a polycrystalline silicon film, high-temperature heat treatment or a laser process which is performed on an amorphous silicon film has been known.
Further, in recent years, an oxide semiconductor film has attracted attention. For example, a transistor which includes an amorphous oxide film containing indium, gallium, and zinc and having a carrier density less than 1018/cm3 is disclosed (see Patent Document 1).
An oxide semiconductor film can be formed by a sputtering method, and thus is suitable for a transistor in a large-sized display device. Moreover, a transistor including an oxide semiconductor film has a high field-effect mobility; therefore, a high-performance display device where driver circuits are formed over the same substrate can be obtained. In addition, there is an advantage that capital investment can be reduced because part of production equipment for a transistor including an amorphous silicon film can be retrofitted and utilized.
In addition, according to Patent Document 2, a transistor including an oxide semiconductor film can have an extremely low off-state current and can be favorably used for a semiconductor integrated circuit owing to such a low off-state current.
As a method for providing a transistor including an oxide semiconductor film with stable electric characteristics, a technique for doping an oxide semiconductor film with oxygen is disclosed (see Patent Document 3). With the technique disclosed in Patent Document 3, the impurity concentration and oxygen vacancies in the oxide semiconductor film can be reduced. As a result, variation in electric characteristics of the transistor including the oxide semiconductor film can be reduced and reliability can be improved.
A technique for reducing contact resistance by providing a buffer layer having low resistance between a source electrode (or a drain electrode) and the oxide semiconductor film to improve the on-state characteristics of a transistor including an oxide semiconductor film is also disclosed (see Patent Document 4).